The Questions of Developmental Biology

(Figure 7.34; Elinson and Rowning 1988). Treating the egg with colchicine or ultraviolet
radiation at the beginning of rotation stops the formation of these microtubules, thereby inhibiting
the cytoplasmic rotation. Using antibodies that bind to the microtubules, Houliston and Elinson
(1991a) found that these tracks are formed from both sperm- and egg-derived microtubules, and
that the sperm centriole directs the polymerization of the microtubules so that they grow into the
vegetal region of the egg. Upon reaching the vegetal cortex, the microtubules angle away from
the point of sperm entry, toward the vegetal pole.
The off-center position of the sperm centriole as it
initiates microtubule polymerization provides the
directionality to the rotation. The motive force for
the rotation may be provided by the ATPase
kinesin. Like dynein and myosin, kinesin is able to
attach to fibers and produce energy through ATP
hydrolysis. This ATPase is located on the vegetal
microtubules and the membranes of the cortical
endoplasmic reticulum (Houliston and Elinson
1991b).
The movement of the cortical cytoplasm
with respect to the inner cytoplasm causes profound
changes within the inner cytoplasm. Danilchik and
Denegre (1991) have labeled yolk platelets with
Nile blue and watched their movement by
fluorescent microscopy (the bound dye fluoresces
red). During the middle part of the first cell cycle, a
mass of central egg cytoplasm flows from the presumptive ventral (belly) to the future dorsal
(back) side of the embryo (Figure 7.35). By the end of first division, the cytoplasm of the
prospective dorsal side of the embryo is distinctly different from that of the prospective ventral
side. What had been a radially symmetrical embryo is now a bilaterally symmetrical embryo. As
we will see in Chapter 10, these cytoplasmic movements initiate a cascade of events that
determine the dorsal-ventral axis of the frog. Indeed, the parallel microtubules that allow these
rearrangements to stretch along what will become the dorsal-ventral axis of the frog (Klag and
Ubbels 1975; Gerhart et al. 1983).